package leetcode.day_2020_2021_2_1_2021_2_7;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

/**
 * 17. 电话号码的字母组合
 * https://leetcode-cn.com/problems/letter-combinations-of-a-phone-number/
 */
public class LeetCode0017 {
    public static void main(String[] args) {
        System.out.println(letterCombinations(""));
    }

    public static List<String> letterCombinations(String digits) {
        List<String> combinations = new ArrayList<>();
        if (digits.length() == 0) {
            return combinations;
        }
        Map<Character, String> phoneMap = new HashMap<Character, String>();
        phoneMap.put('2', "abc");
        phoneMap.put('3', "def");
        phoneMap.put('4', "ghi");
        phoneMap.put('5', "jkl");
        phoneMap.put('6', "mno");
        phoneMap.put('7', "pqrs");
        phoneMap.put('8', "tuv");
        phoneMap.put('9', "wxyz");
        backtrack(combinations, phoneMap, digits, 0, new StringBuffer());
        return combinations;
    }

    public static void backtrack(List<String> combinations,
                                 Map<Character, String> phoneMap,
                                 String digits,
                                 int index,
                                 StringBuffer combination) {
        if (index == digits.length()) {
            combinations.add(combination.toString());
        } else {
            char digit = digits.charAt(index);
            String letters = phoneMap.get(digit);
            int lettersCount = letters.length();
            for(int i=0; i<lettersCount; i++) {
                combination.append(letters.charAt(i));
                backtrack(combinations, phoneMap, digits, index+1, combination);
                combination.deleteCharAt(index);
            }
        }
    }
}
